The best known precedent of this invention is the integrating sphere, also called Helmholz sphere, which is widely used in optical apparatus. The integrating sphere consists of a spherical surface internally covered with an absorber and having a hole, or entry aperture, through which the radiation enters into the cavity. Radiation which has entered the cavity has many possibilities to be absorbed, even if the absorber is poor, because the probability that the light will escape from the cavity through the aperture is small.
A typical application of the integrating sphere is as a black absorber, often done by painting the internal wall of sphere black.
These spheres are also used for purposes other than to enhance light absorption. For instance, they can be internally painted white to produce homogeneous, isotropic illumination on a receiver.
If a certain radiant power is to be absorbed, it must enter entirely through the small entry aperture, which requires a highly concentrated radiant flux at the aperture. This poses many problems, such as expense and excessive heating of the materials used at the entry aperture.
The use of this type of cavity for photovoltaic applications has been proposed by Swanson (U.S. Pat. No. 4,234,352). In this patent the light from a concentrator is cast on a receiver located inside one of such integrating spheres. The receiver becomes hot and emits light with a spectrum cooler than that of the sun, which is better for efficient conversion by a solar cell. Furthermore, light not absorbed by the cell is re-injected into the cavity and again heats the absorber. Because the only optical losses are those in the solar cells, partly recovered as useful electric energy, the losses in the cavity walls and the light escaping through the cavity entry aperture must be kept as small as possible. This requires a high irradiance at the entry aperture from the external concentrator.
Another precedent is the device proposed by R. A. Sinton and R. M. Swanson (IEEE Electron Device Letters, ED-8, 11, pp 547-549, 1987 and in the U.S. Pat. No. 4,960,468) consisting of an internally mirrored box, one of the faces of which has an opening through which the light enters into the cavity and a solar cell located in front of it. The light reflected by the cell, due to reflection on the metal fingers located on top of it for effective current collection, or by the semiconductor itself, is reflected back almost totally and so increases the light collection. In some of the examples, the cavity is hemispheric and the solar cell covers the equator of the hemisphere. However, the only reduction in the loss of light that escapes through the opening is by decreasing the opening size.
We have proposed (A. Luque, J. C. Minano, G. L. Araujo, G. Sala and A. Cuevas, in Proc. 9th EC Photovoltaic Solar Energy Conference, Kluwer Academic, Dordrecht, pp. 802, 1989) a device consisting of a classical integrating sphere covered internally by solar cells to enhance the absorption in the way mentioned above. In a second device proposed there, some of the cells covering the inner surface of the integrating sphere are of a certain type, for instance silicon, and the other cells are another type, for example GaAs. The cells are covered with filters such that photons not absorbed by GaAs are reflected by the GaAs cell and photons absorbed by GaAs are reflected by the Si cell. In this way an efficient spectrum splitting scheme is produced which can also be extended to cells of more bandgaps.
In all known prior cases, the opening of the cavity must be rather small. For example, in the device of U.S. Pat. No. 4,960,468 the opening must be less than 0.2 times the cell area, implying a high concentration at the entry aperture which is often achieved with a secondary attached to the cavity aperture.